Molecular Structural Insight into the Cold Crystallization Process of Ionic Liquid Crystals

We have demonstrated a modified crystallization process of an ionic liquid crystal, 1-{[4'-(4 ''-nitrophenylazo)phenyloxy]}hexyl- 3-methyl-1H-imidazol-3-ium tetrafluoroborate, by the addition of carbon nanotubes (CNTs), successfully suppressing the supercooled state, and hence the cold crystallization during reheating, to the complete conversion of melt crystallization upon cooling. The enthalpies of fusion of the crystals produced were similar among the samples (29.3-32.2 kJ mol(-1)); however, a crystallization-promoting pathway that increases the crystallization enthalpy by 20% was present. Through comprehensive thermal analysis, structural analysis, and infrared spectroscopy combined with quantum chemical calculations, we propose that this effect is caused by an increased crystal nucleation rate due to the pi-pi interactions between the molecule and the rigid CNT surface. These results not only give insights into the crystallization processes of ionic liquids in general but also provide guidelines to expand the range of applications of cold-crystallizing ionic crystalline materials for thermal energy storage applications.